How to soften the edges/faces of a solid?
What you can do is select the left and right edge, as well as the top and bottom loop like so (make sure to do so while selecting edges, not faces):



Then, click N to bring up a tab on the right. Under Transform, set the mean crease to 1 like so:



Then, add a Subdivision Surface modifier to your object:



And you're done, your model should look something like this:



The smoother you want the object to be, the higher the View and Render number in the modifier should be.